Drupal - Disable Rich Text in comment field

Go to URL as in fig (for page comments on page content type). Change the type of text to Plain text as shown.
You can change the comment settings for article or book in similar way by just replacing page from the URL with the respective content types.

The best and the most easy way is to go to the content type For example if the content type is "Articles" then go to Home » Administration » Structure » Content types » Article » Comment fields

And there in the comment field make sure it is set to plain text. And then there will be no problem and the user won't be asked to select a text format from the Drop down. As simple as that.